Second order differential microphone



May 15, 1951 A. M. WlGGlNS 2,552,378

sacoun ORDER DIFFERENTIAL MICROPHONE Filed Sept. 24, 1947 5 Sheets-Sheetl A TTO/WVEYJ y 15, 1951 A. M. WIGGINS SECOND ORDER DIFFERENTIALMICROPHONE 3 Sheets-Sheet 2 Filed Sept. 24, 1947 TIEE! INVENTOR. 41PM?A7 Mam/vs WYW A TTOFNEYS y 1951 A. M. WIGGINS 2,552,878

SECOND ORDER DIFFERENTIAL MICROPHONE Filed Sept. 24, 1947 3 Sheets-Sheet3 INVENTOR.

41/ /74 A7 Maw/vs MYW Patented May 15, i951 UNITED STATES PATENT OFFICESECOND ORDER DIFFERENTIAL MICROPHONE Alpha M. Wiggins, Clay Township,St. Joseph County, Ind., assignor to Electro Voice, Inc., Buchanan,Mich., a corporation of Indiana This invention relates to improvementsin diiferential microphones.

Differential microphones as here considered are of the general typeillustrated in and disclosed in the Patent No. 2,350,010 granted May 30,1944., to F. C. Beekley. A microphone of this type may be defined as apressure gradient microphone which has two or more sound entrancesspaced by an acoustic distance which is small compared to the wavelengths of sound which may pass through said entrances to impinge uponsound responsive generating means incorporated in the microphone wherebythe resultant of the forces acting on the generating means in responseto random sounds of distant origin are attenuated While the resultant ofthe forces acting on the generating means in response to sounds of closeorigin is preponderantly that resulting from the sound pressure at theentrance nearest to the sound source. In other words, a differentialmicrophone is one having a proximity effect rendering the microphonehighly sensitive to sounds of close origin directed preponderantly atone sound entrance and much less sensitive to random sounds of distantorigin so that sounds of close origin are transmitted intelligibly andrandom sounds of distant origin are substantially ineffective upon thegenerating means or have such diminished eifect thereon as not todetract materially from the intelligibility of the translation of thesounds of close origin by the microphone.

The primary object of this invention is to provide a novel, simple andinexpensive microphone of this character which has a very highdiscrimination against random sounds of distant origin and which isresponsive only to the difference between two differential resultantforces of pressure gradients.

A further object is to provide a difierential microphone having adiscrimination of the second order which requires only one soundresponsive generating element therein.

A further object is to providea microphone of this type having two soundsensitive elements, each having two surfaces exposed to sounds travelingthrough different paths wherein said elements are spaced apart and eachhas one surface accessible to sounds having a common path between saidelements whereby sounds of close origin traveling through said commonpath and impinging on said elements produce an output substantiallydouble the output of the microphone compared to that of a microphonehaving a similar single sound sensitive element.

A further object is to provide a microphone having at least two similarvibrating elements whose axes are parallel and which are so ar-- rangedand related to a generating element that movements of said elementswhich are of equal amplitude and in phase produce or generatesubstantially zero voltage in the generating element. The axis of avibrating element of a microphone is considered to be a linerepresenting the vectorial sum of all lines connecting the centers ofthe sound access openings of the microphone at opposite sides of saidelement.

Other objects will be apparent from the following specification.

In the drawing:

Fig. 1 is a perspective view illustrating one embodiment of theinvention.

Fig. 2 is a sectional view taken on line 22 of Fig. 3.

Fig. 3 is a face view of the central portion of the housing of themicrophone shown in Fig. 2.

Fig. 4 is a vector diagram illustrating the resolution of the forces inthe microphone.

Fig. 5 is a sectional view similar to Fig. 2 and illustrating theembodiment of the invention in a sound power type of microphone.

Fig. 6 is a sectional view similar to Fig. 2 and illustrating theembodiment of the invention in a condenser type of microphone.

Fig. 7 is a sectional view similar to Fig. 2 and illustrating theembodiment of the invention in a dynamic or moving coil type ofmicrophone.

Fig. 8 is a schematic view illustrating the arrangement of the movingcoil with respect to the magnet of a magnetic type of microphone.

Fig. 9 is a sectional view similar to Fig. 2, illustrating theembodiment of the invention in a microphone having a piezo electriccrystal type of generating element.

Fig. 10 is a sectional view taken on line Ill-40 of Fig. 9.

Fig, 11 illustrates the response characteristics of one embodiment ofthis invention.

Fig. 12 is a schematic view illustrating the arrangement of a pair ofvibrating elements in a device of this character to produce the responsepattern illustrated in Fig. 11.

Fig. 1-3 is a response pattern of another embodiment of this invention.

Fig. 14 is a diagrammatic view illustrating the relation of thevibrating elements to produce the response pattern shown in Fig. 13.

Fig. 15 is a response pattern of another embodiment of this invention.

Fig. 16 is a diagrammatic view illustrating the arrangement of thevibrating elements to produce the response pattern shown in Fig. 15.

Referring to the drawings, and particularly to Figs. 1 to 3 wherein amicrophone of the carbon granule type is shown, the numeral 20designates a housing which preferably includes an annular center portion22 which preferably has a spider'formed therein in the nature ofinwardly projecting arms 24 terminating in a concentric annular portion26. The annular poition 26 is of a thickness small compared. to theaxial dimension of the portion 22 and is positioned centrally withrespect to the longitudinal dimension of the housing portion 22. Thean.- nular member 26 is preferably provided with a metal sheath or ring28 of channel shape in cross-section, which fits around said'ring 28 asillustrated in Fig. 2. The interior wall surface of the housing portion22 is preferably of stepped form, as illustrated at 30, to providesimilar seats at its opposite margin for the reception of gaskets 32which are adapted to engage diaphragms or other sound sensitive elements32. Each of the diaphragms in this embodiment of the inventionpreferably has a central conical portion or button 34 projectinginwardly as shown and is similar to or balanced with respect to theother. A pair of gasket rings 36 which are preferably positioned byflanges 38 on the metal ring 23 span the space between central housingring 26 and the diaphragms 32 to cooperate with the member 23 and thediaphragms 32 for the purpose of enclosing a space centrally of themicrophone within which carbon granules is may be retained. Anelectrical lead 42 is connected with each of the twodiaphragms 32.

The casing is completed by two similar end members 44 which are ofsubstantial cup-shape form'and each of which has an end wall 46 and amarginal flange 40. A gasket 50 fits in each of the end members 44 andbears against the outer surface of the adjacent diaphragm 32. Thecentral housing portion 22 has one or more sound access openings 52formed therein centrally between the two diaphragms 32, and each of theend portions 44 of the housing has one of the openings 54 and 56 formedtherein preferably at the center thereof. The various parts of thecasing are secured together by suitable securing means such as thescrews 58 shown in Fig. 1.

The housing may be formed of any suitable material such as metal orplastic and is preferably molded from plastic material. The size of thehousing is small so that the spacing between the openings 52, 54, 56 issmall compared to the Wave length of the sound of highest frequencywhich is intended to be transmitted. In the preferred embodiment, thespacing between the openings 52, 54 and 56 will preferably be in theorder of three-eighths of one inch when the device is designed for useunder conditions where random sound will include sounds havingfrequencies as high as 10,000 cycles. It will be understood, however,that random sounds in noisy and reverberatory locations, such as mill-'tary tanks, airplanes, ships, machine shops, or

other locations where the device maybe used, seldom include sounds offrequencies as high as 10,000 cycles, and such random sounds arecommonly of much lower frequency. Consequently, a dimension ofthree-eighths of an inch spacing between openings representssubstantially the optimum condition for cancellation of random be met.

sounds. Practical constructions of the microphone may include unitswhere the spacing between the openings is increased above threeeighthsof an inch as long as the spacing dimension remains small compared tothe wave length of the highest frequency likely to be encountered in therandom sound intended to be cancelled by the device. In cases wheresound components at the location for which the microphone is designedare known to have a certain maximum frequency, the microphone may bedesigned for effective use in that location by holding the spacingbetween the openings or sound passages to 'a dimension not'substantiallyexceeding onesixth of the wavelength of random sound of the highestfrequenc being encountered.

It will be understood that, while the invention shown in Fig. 2illustrates two diametrically opposed central sound openings andcentrally positioned end sound openings 52 and 56, only three soundopenings need be employed, namely, one opening 52 and one each of theopenings 54 and 56. Also these openings may be positioned close togetherb arranging the openings 54 and 56 eccentrically with respect to thecasing walls 46 as long as the requirement of a similar relation of eachof said openings 54 and 58 with respect to the chamber or cavity intowhich the same leads is maintained. It will be understood further thatthe two diaphragms 32 should be of balanced construction and that thecavities or chambers whose inner walls constitute said diaphragms are ofsimilar size, shape and volume.

It will be observed that both of the diaphragms 32' in this device areopen at both the front and the back thereof. Each of these diaphragms isresponsive to a difference in pressure between the front and the backthereof. In other words, sound pressures acting upon opposite faces ofeither of the diaphragms and which are of equal amplitude and in phasewill not energize the diaphragm. The sound pressures at the soundentrances due to random sound are of equal amplitude due to the sourceof the sound being at a distance. There will be a phase difference,however, due to a shift in phase as the sound travels the distancesbetween the sound entrances. The instantaneous difference between thesound pressure at one opening to a diaphragm and the sound pressure atthe other opening is the pressure gradient. For sounds originating veryclose to one sound opening, the pressure at one opening is of greateramplitude than the sound pressure at the other. The pressure gradient istherefore greater for sounds of close origin than for sounds of distantorigin causing a diaphragm which is open to the atmosphere on both sidesto be more responsive to close sounds than to distant sounds. By virtueof the distance the requirement of equal amplitude will automatically Inorder to energize either diaphragm there must be a pressure gradientbetween the actions upon the opposite faces of the diaphragm. It mightbe mentioned in this connection that where the spacing between theopenings is in the order of three-eighths of an inch, the sound sourceshould be very close. A microphone of this type is the U. S. Army T-45microphone which is positioned directly against the face of the userwith one of the sound openings directly in front of the lips of theuser. In instances Where the spacing is greater than this minimumspacing of three-eighths of an inch, the sound source maybe located atsomewhat greater distance from the microphone and still produce thepressure gradient necessary to energize the device. In the instantdevice the combination of the two diaphragms arranged as shown, each ofwhich is responsive to the gradient of pressure as explained above,produces a response which is the gradient of the gradients of pressureof the two component diaphragm units. In other words, the response ofthe microphone is of the second order. The response characteristics areillustrated graphically in Fig. 4, wherein Pl will represent thepressure of the sound passing through one of the openings, for instanceopening 54, P2 and P3 represent the pressure of sound acting upon thetwo diaphragms at the inner faces thereof and entering the microphonethrough the opening or openings 52, and P4 represents the pressureacting upon the microphone and entering through the passage 56. Fordistant sounds the pressures are all of equal magnitude and the vectorsPl, P2, P3 and P4 are thus illustrated as of equal length. These distantsounds are, however, displaced in phase by an angle D where k is k isthe wave length of sound, and D is the acoustic distance between thesound openings 52, 54 and 56. The pressure available for actuating oneof the diaphragmsfor instance the upper diaphragm in Fig. 2, isdesignated PI and is the vector difference between the two pressuresacting on the upper diaphragmas shown in Fig. 4. Similarly, the pressureavailable for actuating the other of the two diaphragms, which, isdesignated P2, is the vector difference between the two pressures actingon the lower diaphragm as shown in Fig. 2. The pressure P0 which isavailable for actuating the carbon element 54 is the vector differencebetween these two available pressures (PIP2). It will be observed thatPD is very small by comparison to the pressures Pl, P2, P3 and P4, andalso is very small in comparison with the pressure gradients PI and P2,hence the vector P0, which represents the sensitivity of the microphoneor its response to sounds of distant origin, is very small andrepresents the very low transmission of random sound by the microphone.This represents a very high discrimination against random sounds andreduces the transmission of such sounds to a factor negligible comparedto that existing in such previous differential microphones as the ArmyT-45 microphone mentioned above. 4

Transmission of sounds or signals by the microphone is limited to soundsoriginating at close origin. For this purpose the microphone may beoperated by talking into any one of the openings 52, 54 or 56.Preferably, the microphone will be actuated by talking into the opening52 between two diaphragms, thereby actuating the dia-' phragms inopposite directions and securing a greater output of the sounds intendedto be transmitted. The sounds of close origin are translated with highfidelity and their intelligibility is very high because of the greatlydiminished force P0 which represents the translation of the randomsound. Thus it is possible by using a microphone of this type in thenoisiest locations, which. in a military tank often rise to a level ofintensity of random or background sounds which can be described as anintensity at the threshold of pain, to be assured of high fidelity andhigh intelligible performance. In other words, when a trans mitter islocated in a tank, the speech of the operator will be intelligible at areceiving station, and the random or background noise within the tankwhich is at the high level mentioned above will be negligible insofar asits transmission by the microphone and insofar as the signal at thereceiver is concerned. It will be understood that the condition of usewithin a military tank represents an extreme condition, and further thatreference to use in a miltary tank are illustrative only. Comparableresults of high intelligibility of transmission of intended signals willobviously result when the microphone is operated in other noisylocations, as in factories, noisy public buildings such as railroadstations, and the like.

In addition to the diagrammatic or vectorial illustration of theoperation of the microphone, the response pattern of the microphone hasbeen illustrated in Figs. 15 and 16. Fig. 16 represents the arrangementof the acoustical axes of the two diaphragms as coincident, a conditionwhich-is.

satisfied in the embodiment of the invention shown in Figs. 1, 2 and 3,it being understood that the axes of the diaphragms are determined asmentioned hereinabove. Fig. 15 illustrates the polar response pattern ofthe microphone, which in this instance is shown as cos 0, where 0 is theangle of incidence.

The same principle may be provided in a. microphone of the sound powertype as illustrated in Fig. 5. In this construction a housing 60 has twodiaphragms 62 and 64 mounted therein in offset relation but with theiraxes parallel. The housing 50 has an opening 66 leading to a chamber 63defined in part by one face of the diaphragm B2. A similar opening H1 isformed in the housing 60 leading into a chamber '52 defined in part bythe diaphragm 64. The two chambers 68 and 12 are similar. One or moreopenings 14 in the casing lead into the cavity defined in part by theinner faces or surfaces of the diaphragms 62 and 64.

A permanent magnet 15 is suitably mounted in the housing 6G and has twosets of poles, which pole sets are spaced apart as illustrated. Thepoles, namely an inner set of poles 18, T9 and an outer set of poles 80,8|, are so arranged that the two poles on the same leg of the permanentmagnet, here shown as of U-shape, are of the same polarity. In otherwords, pole pieces: 18 and are of one given polarity and pole pieces '19and 8| are of opposite polarity. A steel armature 82 of elongated form,as shown, passes through the magnetic fields of the two sets of polepieces l8, l9 and 80, 8| as shown, and preferably is positionedcentrally with respect to both sets of pole pieces. A link 84 connectsone end of the armature 82 with the diaphragm. 62, and a link 86connects the other end of the armature 82 with the diaphragm 54. A voicecoil 88 is wound upon the armature between the two sets of pole pieces.

, It will be apparent that the same response of the microphone of Fig. 5to distant sounds, as has been illustrated by the vector diagram in Fig.4, will result from the operation of this construction of microphone, sothat the very low Pi! output of this sound power embodiment in responseto distant sounds will substantially reduce the transmission value ofsuch sounds. This form of microphone, like the preferred form, isrendered operative to transmit a desired signal by talking close to oneof the openings 56, 7!], E4 to secure a large pressure gradient actiondue to the difference in pressure resulting from application of thesignal preponderantly through. one of said openings. As in theembodiment previously discussed, maximum output of the signal desired tobe. transmitted is obtained by speech through the opening 14 betweenthe'diaphragms which doubles the output as compared to other microphoneshaving similar components and as com; pared to speech directed, at oneof the openings 66 or H! of this embodiment.

The microphone shown in Fig. has a polar response pattern as illustratedin Fig. 13 by rea son of the offset relation of the acoustical axes ofthe diaphragms 62 and 64 as shown in Fig. 14. Notice in this instancethat the axis of the microphone, considered as a whole and indicated atX in Fig. 14, is disposed at an angle to the axes Y of the individualdiaphragms 62 and 64. The polar response of this microphone is cos (0)cos (0+X), where X is the angle of displacement of the axis X of themicrophone as a whole compared to the axis Y of the individualmicrophones. It. is essential for satisfactory operation of this devicewhere the X and Y axes are angularly disposed, that the Y axes of theindividual diaphragms shall be parallel to each other as illustrated.

The application of the invention to a microphone of the condenser typeis illustrated in Fig. 6, wherein the casing 90 has a centrallypositioned condenser plate 92 which is provided with openings 94therein. Two diaphragms 96 and 93 are mounted in the casing in closespaced capacitative relation to the element 92. The casing 00 has anopening I00 leading into the chamber I02 defined in part by the outerface of the diaphragm 98, and also has a similar opening 504 leadinginto a chamber Q06 defined in part by the outer face of the diaphragm98. The chambers I02 and I06 are similar and the openings H10 and I04are similar and similarly located. An opening I08 is formed in thehousing and communicates with the space between the diaphragm 06 andcondenser element 92, and a similar opening H0 is formed in the housingand communicates with the space between the diaphragm 00 and thecondenser element 92. It will be understood that. the spacing betweenthe two diaphragms and the condenser element 92 is the same.

The response of this microphone to sound of distant origin is the sameas that illustrated diagrammatically in Fig. 4, and the polar responsepattern of this microphone is the same as that illustrated in Figs. and16 by virtue of the coincidence of the acoustical axes of the individualmicrophones with the axis of the microphone considered as a whole. Inthis case electric leads I I2 will extend from the two diaphragms 96 and98.

Fig. '7 illustrates the embodiment of the invention in a dynamicmicrophone. A casing I20 mounts diaphragms I22 and I24 in ofisetrelation. An opening I26 leads into a chamber I28 defined in part by theouter face of the diaphragm I22, and a similar opening I30 leads into achamber I32 defined in part by the other face of the diaphragm I24.Chambers I28 and I32 are similar, and the openings I26 and I30 aresimilar and are located in the same relation-to said chambers.

A magnet I34 having pole pieces I36 terminating in spaced confrontingrelation and also having a pole piece I38 positioned between the polesI36, is suitably mounted in the center of the housing I20. A coil I40 ofa shape to extend freely around the pole piece I38 and between the polesI36 and the pole pieceI38 is mounted at 8, opposite ends thereof as atI42 to the diaphragms I22 and. I24.

If the voice coil I40 moves equally and similarly relative to the polesI36-and pole pieces I38, the voltage between one pole I36 and pole pieceI30 cancels the voltage between the other pole I36 and pole piece I38.The voltage output of this microphone will be proportional to thevectorial difference between the velocities of the two diaphragms I22and I24, as illustrated in the vector diagram in Fig. 4. The polarresponse pattern of this form of microphone will be a response asillustrated in Fig. 13, by reason of the location of the acoustical axesof the two constituent diaphragms in offset relation a illustrateddiagrammatically in Fig. 14. In-this instance it will be understood thatit is necessary for the axes of the two constituent diaphragms to beparallel.

Fig. 8 illustrates the application of the principle of this invention toa magnetic microphone, wherein it will be understood that the samearrangement of diaphragms, chambers and openings as illustrated in Fig.7 will be employed and wherein similar parts bear the reference numeralsas Fig. '7.

A crystal microphone may be constructed, in accordance with thisinvention as illustrated in Figs. 9 and 10. In these figures a casing Iencases a crystal piezoelectric element I52. The crystal I52 ispreferably mounted at its mid point as by the mounting member I54. Adiaphragm I55 is mounted in the housing and is connected at'its centerto two diagonally opposed corners of the crystal I52 by a bridge memberI56. A second diaphragm I is mounted'in the housing and is connected tothe crystal by means of a bridge member I62 which spans the crystal toconnect diagonally opposed portions thereof and which is positioned atan angle to the bridge I58, as best shown in Fig. 10. An opening I64 inthe housing communicates with a chamber I66 defined in part by thediaphragm I56, and a similar opening I68 in the housingleads to achamber I10 defined in part by the diaphragm I60. The chambers I 66 andIll] are similar, and the openings I64 and I68 are similar and similarlyarranged. One or more openings I12 are formed in the housing tocommunicate with the space therein between the two diaphragms and leadsIl extend from the piezoelectric element I52. The response of thismicrophone to sounds of distant origin is the same as illustrated in thevector diagram in Fig. 4. The polar response r pattern of thismicrophone corresponds to that illustrated in Fig. 15 by virtue of thecoincidence of the acoustic axis of the microphone as a whole with theacoustic axes of the two constituent diaphragms I66 and I60, asillustrated in Fig. 15.

While the microphones shown herein are all arranged with the acousticaxes thereof in one of the two relations with respect to the acousticaxes of the constituent diaphragms Which are illustrated in Figs. 14 and16, such arrangements are not critical in the device, and an arrangementof the diaphragms I and I32 in a common plane, as illustrated in Fig.12, is possible. Where such an arrangement is utilized, the acousticaxes of the two diaphragms E80 and I82 must be parallel to each other.The axis of the second order result or vector secured by a microphone ofthis character, where the openings 184 leading to the .two diaphragmsI80 and I62 are arranged as illustrated diagrammatically in Fig. 12, isat right angles to the axes of thet'wo diaphragms themselves as isillustrated at Z in Fig. 12. The openings I84, will, of course, bearranged so that the space therebetween is small compared to thewavelength of the sound of highest frequency occurring in random soundin order to secure the differential action explained above and an actionwhichcorrespond with or is represented by the vector diagram shown inFig. 4. The polar response of a microphone whose elements are arrangedas illustrated in Fig. 12 is shown in Fig. 11. This polar response iscos sin 0.

While the preferred embodiments of the invention have been illustratedand described herein, it will be understood that changes may be madetherein within the scope of the appended claims without departing fromthe spirit of the invention.

I claim:

1. A microphone comprising two vibrating elements each element having aninside and outside surface, means whereby a pair of acoustic networksare associated with each element to render each element responsive tothe difference in pressure acting on its inside and outside surface, andmeans for attaching a common transducer element subtractively to bothvibratory elements whereby -no voltage is generated therein when saidvibratory elements move with the same amplitude and in phase.

2. A microphone comprising two vibratory elements, a pair of acousticnetworks associated with each element, means whereby each element isresponsive to the difference in pressure acting on opposite surfacesthereof, and means whereby a common electric generating element issubtractively attached to both vibratory elements whereby no voltage isgenerated therein when said vibratory elements move with the sameamplitude and in phase, said networks having inlets spaced apart adistance which is small compared to the wave length of sound of thehighest frequency desired to be attenuated whereby random soundsoriginating at a distance and passing through said networks forimpingement on the surfaces of said Vibratory elements with equalamplitude is only displaced in .phase at said surfaces.

3. A differential microphone comprising a housing, a pair of vibratoryelements dividing said housing into at least three chambers, an openingmeans leading into each chamber whereby each element is responsive tothe difference in sound pressure at its opposite faces, and meanswhereby an electric generating element is connected to said vibratoryelements in subtractive relation for response to the difference betweenthe resultant differential forces acting on the two vibratory elements,said openings being spaced apart a distance small compared to the wavelength of sound of the highest frequency to which said elements respond.

4. A differential microphone as defined in claim 3, wherein said openingmeans are so oriented relative to said vibratory elements that theacoustic axes of said vibratory elements are parallel.

5. In a transducer, a pair of similar spaced diaphragms, each having aninside and outside face, means whereby each diaphragm is exposed toatmosphere at each of said inside and outside faces, and means whereby asingle electric generating element is subtractively connected to andpositioned between said diaphragms whereby no voltage is developed insaid element when said diaphragms move with equal amplitude and inphase.

6. A differential microphone comprising a housing, a single soundresponsive electrical generating means in said housing, and two similarvibratory sound responsive members, said generating means beingpositioned between said vibratory members, and means rendering saidgenerating means oppositely responsive to said vibratory members, saidhousing having a plurality of opening means located to render allsurfaces of both vibratory members substantially. equally responsive torandom sounds of distant origin and to define the acoustic axes of saidvibratory members in parallel relation.

'2'. A differential microphone comprising a housing, a pair of vibratorymembers mounted in said housing and each having two sound sensitivesurfaces, and a single sound responsive generating element connected foractuation by said members in opposition, said housing having a pluralityof openings each accommodating passage of sound .therethrough forimpingement upon one of the sound sensitive surfaces of one of saidmembers to render said members responsive to the difference in sound atopposite faces thereof, said members providing two sound responsivesystems each having said generating element as a part thereof, whereinsaid openings are located to define parallel acoustic axes for saidsystems.

8. A differential microphone comprising a housing, an electricgenerating element in said housing, a pair of diaphragms mounted in saidhousing in offset relation and connected to said element to energize thesame oppositely, and openings in said housing for passage of sound intothe housing between said diaphragms and outwardly of said diaphragmsrespectively, and

means whereby said openings are arranged rela tive to each other and tosaid diaphragms so as to admit sound of distant origin. in equalamplitude and. substantially in phase and to provide an acoustic axisfor each diaphragm parallel to the acoustic axis of the other diaphragm.

9. A differential microphone comprising a phragms in the space definedby said retainer and confining rings, said housing having opening meanstherein for passage of sound from a distant origin to impinge upon eachof the faces of said diaphragms with equal amplitude and only slightlydisplaced in phase with respect to sounds of the highest frequencyeifective to actuate the diaphragms and carbon granules includingopening means leading to the space between said diaphragms.

10. A differential microphone comprising a housing, a magnet having twospaced sets of confronting pole pieces separated by a gap, the polepieces on one side of said gap being of the same polarity, an armaturepositioned in said gap, a stationary voice coil encircling saidarmature, and a pair of diaphragms having inner and outer surfaces andconnected to opposite ends of said armature, said housing having aplurality of openings each directing sound to one of the surfaces of adiaphragm, some of the openings 11 leading to. the inner diaphragmSurfaces and other leading to the outer diaphragm surfaces. I 11-. Adifferential microphone comprising a housing, a magnet havin'g'twospaced sets of confronting pole pieces separated by a gap, the polepieces on one side of said gap being of the same polarity, an armaturepositioned in said gap, a stationary voice coil encircling saidarmature, and a pair of diaphragms having inner and outer surfaces andconnected to opposite ends of said armature, said housing having aplurality of openings each directing sound to one of the surfaces of adiaphragm some of said openings leading to the inner diaphragm surfacesand others leading to the outer diaphragm surfaces, and means wherebysaid diaphragms are positioned in offset relation in said housing and"so related to the openings for directing sounds to the opposite facesthereof that the acoustic axes'of said diaphragms are parallel.

12; A microphone comprising a housing, a pair of diaphragms mounted insaid housing, a single generating element interposed between saiddiaphragms and having an actuating conn'e'cticn with each diaphragm, aplurality of openings in said housing, means whereby one surface of eachdiaphragm is responsive only to sound entering a selected passagecorrelated therewith, means whereby said openings position the acousticaxes of said diaphragms in parallel relation and means connecting saiddiaphragms tosaid generating element in such a manner that the voltagegenerated by said element upon movement of said diaphragms of equalamplitude and in phase is substantially zero.

13. A transducer as defined in claim 5, wherein said electric generatingelement constitutes a mass of carbon granules interposed between andcontacting 'said diaphragms, and means for retaining said carbongranules between said diaphragms.

14. A transducer as defined in claim 5, wherein said electric generatingelement constitutes a magnet having two spaced sets of spaced poles, anarmature positioned within and passing through the "magnetic field ofthe two sets of poles, means connecting the ends of said armature to theopposite diaphragms, and a voice coil wound upon said armature.

1'5. 'Atr'ansducer as defined in claim -5,rwherein 'said'electricgenerating element constitutes an apertured condenser platemountedmidway .between and in close spaced capacitative relation to saiddiaphragms, said diaphragms constituting condenser plates. 7

16. A transducerasdefined in claim'5, wherein said electric generatingelement constitutes a magnet having a pair of spaced confronting .polcpieces, a third pole piece interposed between the first pole pieces, acoil extending freely around the third pole piece and between the firstnamed pole pieces and the third pole piece, and means connecting saidcoil to both diaphragms.

17. A transducer as defined in claim 5, wherein said electric generatingelement constitutes a piezoelectric element, 'means mounting saidpiezo-electric element, means connecting said piezoelectric element atdiagonally opposed portions thereof to one diaphragm, and meansconnecting said piezoelectric'element to the other diaphragm at twodiagonally opposed portions spaced from said first named diagonalportions.

ALPHA M. WIGGINS.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,117,231 PalmerNov. 17, 1914 1,496,919 Bellus June 10, 1924 1,546,749 Roberts July 21,1925 1,753,137 Seibt Apr. 1, 1930 1,930,186 Swallow Oct. 10, 19332,164,157 Kennedy June 27, 1939 2,184,247 'Baumzfweiger Dec. 19, 19392,196,342 Ruttenber'g Apr. 4, 1940 j--2,l98,4 24 Baumzweiger Apr. 2-3,1940 2,233,886 9 Cowley Mar. 4, 1941 2293258 Harry -1 Aug. 18, 19422,295,376 A-nderson Sept. 8, 1942 2,301,744 Olson Nov. 10, .1942"2,476,396 Anderson "July 19, 1949 FOREIGN -PATENTS Number Country Date212,857 Great Britain Mar. 14, 1924

